1. Field of the Invention
This invention relates to a solenoid for an electromagnetic valve which is suitable for use in, for example, an electromagnetic proportional control valve.
2. Description of the Prior Art
In general, a solenoid for an electromagnetic valve comprises an exciting coil, an inner housing excited by the exciting coil, and a plunger made of a magnetic material slidably contained within the inner housing. Accordingly, in order to operate the plunger, it is definitely required to form a small space portion between the inner periphery of the inner housing and the outer periphery of the plunger.
However, it is difficult to adequately set the space portion. For example, if the space portion is made too large, the plunger tends to fall and rub the inner periphery of the inner housing thereby to produce a loss of electromagnetic force, while if the space portion is made too small, there is a possibility that the plunger becomes unable to operate due to dust, etc. which enter therein.
In view of the above, the above-mentioned space portion was heretofore properly set and, as means for holding the space portion, for example, the plunger was provided on both ends thereof with a guide rod of a small diameter projecting therefrom and the rods were slidably held by a bearing portion which was provided coaxial with the plunger. However, the guide rods and the bearing portions were required to be precisely machined. In addition, when these were to be mounted, they were required to be precisely located coaxial with respect to the coil. Accordingly, these problems have been required to be solved.
In recent times, in order to obtain a predetermined result, the said space is formed of a non-magnetic tubular body. However, since a pressure oil of high pressure is usually contained around the tubular body, it is impractical that the tubular body is made of a thin cylindrical member because of lack of strength thereof. As another example, Japanese patent early laid-open publication No. 50-43523 discloses a device in which the tubular body is formed of a thick cylindrical member in order to ensure the strength. In the above-described solenoid, the tubular body can stand the high pressure from the pressure oil because of its thick structure. However, since the plunger is directly slidably disposed within the magnetic portion which is located coaxial with the tubular body, loss of the electromagnetic force is significantly large and therefore this is not effective.